(i) Field of the Invention
The present invention relates to a novel phenolic resin which can be utilized in many fields as a friction material, a molding material, a coating material, a raw material for an epoxy resin, a curing agent for an epoxy compound or the like, and a method for preparing this phenolic resin.
More specifically, the present invention relates to a novel phenolic resin having excellent heat resistance, electrical properties, wear resistance and chemical resistance and the like as well as a good curing reactivity.
(ii) Description of the Related Art
In general, a phenolic resin is a resin having excellent heat resistance, electrical properties, wear resistance and chemical resistance, and thus it plays an industrially important role. In recent years, however, phenolic resins having further improved characteristics are desired with the advancement of techniques in the concerned fields, and various resins having different structures have been suggested.
For example, U.S. Pat. No. 4,318,821 discloses a low-molecular weight polymer named the so-called Friedel-Crafts resin, which can be obtained by a condensation reaction of an aralkyl compound such as .alpha.,.alpha.'-dimethoxy-p-xylene and a phenol.
Furthermore, U.S. Pat. No. 4,340,700 suggests a method for preparing a Friedel-Crafts resin, i.e., a phenol aralkyl resin having a high melting point and a declined cold flow, and this method is characterized by controlling an aldehyde content and an acetal content in an aralkyl ether compound containing 50 mol % or more of a difunctional aromatic compound, such as .alpha.,.alpha.'-dimethoxy-p-xylene and the like in the case of the preparation of the Friedel-Crafts resin by a condensation reaction of an aralkyl compound and a phenol.
However, when this kind of Friedel-Crafts type phenolic resin is reacted with a curing agent, for example, hexamethylenetetramine (hereinafter referred to simply as "hexamine"), the resultant cured resin has more excellent heat resistance as compared with a cured product obtained from a phenolic novolak resin, but the Friedel-Crafts resin has a large drawback that the productivity of the molded products thereof is low, since the curing reactivity of the resin with the hexamine is low.
For the purpose of removing this drawback, U.S. Pat. No. 3,838,103 has been suggested which reveals a method of adding a curing accelerator such as salicylic acid. However, this method brings about blisters on the surfaces of the molded articles sometimes, and it is inconveniently difficult to obtain the molded articles having a uniform shape.
An object of the present invention is to provide a phenolic resin by which the above-mentioned problems can be solved and which has excellent physical properties such as heat resistance, electrical properties, wear resistance and chemical resistance as well as a good curing reactivity, and another object of the present case is to provide a method for preparing the above-mentioned phenolic resin.